Optical velocimeter probe

Abstract

An optical velocimeter probe a device to illuminate a volume of measurement in which there may pass particles, and an optical detector to produce an electrical signal x(t) in response to the passage of a particle in the illuminated volume. It also includes a device to extract information representing the velocity v of particles from the electrical signal, including, in particular, a device to convert the electrical signal x(t) into a complex signal x (t), an apparatus to set up a self-correlation function r x (m) of the signal x(t), and a device to estimate the velocity v from the computation of the phase φ x (m) of the function r x (m).

What is claimed is: 
     
       1. An optical velocimeter probe comprising means to illuminate a volume of measurement in which there may pass particles in relative motion with respect to the probe, and means of optical detection to produce an electrical signal in response to the passage of a particle in the illuminated volume of measurement, the frequency spectrum of this signal having a width centered on a dominant frequency f o  representing the relative velocity of the particle with respect to the probe, this probe lo comprising digital means to set up a vector x(t) representing N digital samples of the electrical signal detected, wherein said probe comprises: means to convert the electrical signal x(t) into a complex signal  x  (t), having a real part and an imaginary part comprising a phase term dependent on f d ,   means to set up a self-correlation function of the complex signal, said function being defined as follows: ##EQU15## where E{x} designates the mathematical expectation of the variable x, means to determine the phase φ x  (m) of the function r x  (m) dependent on the imaginary part of said function;   means to determine the frequency f d  from the set of pairs  m,φ x  (m)!;   means to provide an indication of velocity v of a particle with respect to the probe on the basis of the frequency f d .   
     
     
       2. An optical velocimeter probe according to claim 1, wherein the vector x(t) is defined by the following equation:   x(t)=A.e.sup.-2α.spsp.2.sup.f.spsp.2.sup.t.spsp.2 cos (2πft)+w(t)     where α a is a fixed coefficient, t denotes time,   w(t) is a noise term.     
     
     
       3. An optical velocimeter probe according to claim 1, wherein the means for converting the real signal x(t) into a complex signal  x  (t) comprise Hilbert filter type means. 
     
     
       4. A velocimeter probe according to one of the claims 1 to 3, wherein the means use to define the complex function  x  (t) and the means used to define the function r x  (m) comprise the following means: Means to define the Fourier transform function x(ω) of the function x(t) on N samples;   means to elaborate the function X(ω) such that   x(ω)=2|x (ω)| 2  if ω>0   x(ω)=0 if ω<0   means to define the function r x  (m)=FFT -1   X(ω)!, if FFT -1  designates the reverse Fourier transform.   
     
     
       5. A velocimeter probe according to claim 1, wherein the means to define the frequency f d  include means to search for the frequency value for which the function ##EQU16## is the minimum. 
     
     
       6. A velocimeter probe according to claim 5, wherein the value f o  is estimated by f d  ##EQU17## M being the number of self-correlation points. 
     
     
       7. A velocimeter probe according to one of the claims 1 to 3, wherein the phase φ x  (m) is determined by the element arctan ##EQU18## with Im.r x  (m): the imaginary part of r x  (m) Rer x  (m): the real part of r x  (m).